1. Field of the Invention
The present invention relates to a solid-state imaging apparatus, an imaging system, a driving method of a solid-state imaging apparatus, all used for a scanner, a video camera, a digital still camera, and the like.
2. Description of the Related Art
With regard to a conventional imaging apparatus, a pixel configured so that a photoelectric conversion portion for photoelectrically converting an incident light may be used also as an accumulation portion for accumulating charges has been generally known.
On the other hand, Japanese Patent Application Laid-Open No. 2004-111590 discloses a technique for providing a charge accumulation portion apart from a photoelectric conversion portion. By the technique disclosed in Japanese Patent Application Laid-Open No. 2004-111590, the charges accumulated in all of the pixels in the photoelectric conversion portions are simultaneously transferred from the photoelectric conversion portions to the accumulation portions to enable the realization of a global shutter.
Moreover, Japanese Patent Application Laid-Open No. 2006-246450 discloses a configuration in which photoelectric conversion portions are provided apart from accumulation portions and most of the charges generated in the photoelectric conversion portions are not accumulated in the photoelectric conversion portions but are transferred to charge accumulating regions. FIGS. 9A-9G are quoted from FIG. 6 of Japanese Patent Application Laid-Open No. 2006-246450. The configuration disclosed in Japanese Patent Application Laid-Open No. 2006-246450 uses a metal oxide semiconductor (MOS) transistor of a buried channel structure in a transfer portion between a photoelectric conversion portion and a charge accumulation portion. Japanese Patent Application Laid-Open No. 2006-246450 describes that the photoelectric conversion portion can be restricted to the minimum size necessary for light reception. By this configuration, an in-surface synchronous electronic shutter by which the start times and the finish times of accumulations of all pixels in a surface are made to be uniform can be realized.
FIGS. 10A-10C and 11A-11C are diagrams quoted from FIGS. 2 and 5 of Japanese Patent Application Laid-Open No. 2004-111590, respectively. According to the technique disclosed in Japanese Patent Application Laid-Open No. 2004-111590, a control switch ABG for discharging the charges accumulated in a photodiode, which is a photoelectric conversion portion, to a power source VDD is first set to the off-state thereof. Then, an exposure-and-accumulation period is started at a time t2 in FIG. 11A, which is the timing when the operation of transferring the charges accumulated in the photoelectric conversion portion to an accumulation portion has been completed. After that, after the start of an exposure-and-accumulation period, the charges transferred to the accumulation portion are transferred to a floating diffusion FD.
However, such an operation cannot sometimes define an accumulation period in a pixel accurately. In other words, there is a case where it is impossible to grasp the period accurately in which period the charges to be accumulated in a pixel have been generated. For example, if the case where a photoelectric conversion portion is designed so as to be the minimum size necessary for light reception as disclosed in Japanese Patent Application Laid-Open No. 2006-246450 is considered, then it is apprehended that the charges generated in a photoelectric conversion portion exceed a charge quantity capable of being accumulated in the photoelectric conversion portion in the state illustrated in FIG. 11A. The charges exceeding the charge quantity capable of being accumulated in the photoelectric conversion portion are discharged to the power source VDD or the accumulation portion over the potential barriers formed by the control switch ABG and an FSG. That is, the problem in which an accumulation period cannot be accurately defined because a part of the charges generated after the start of the accumulation period is discharged is caused. Consequently, the driving method disclosed in Japanese Patent Application Laid-Open No. 2006-246450 cannot define the start of an accumulation period accurately in a use of generating charges more than a charge quantity that a photoelectric conversion portion can accumulate and in an imaging apparatus including a photoelectric conversion portion designed to have the minimum size necessary for light reception.
Moreover, in the operation of the configuration disclosed in Japanese Patent Application Laid-Open No. 2006-246450, which operation is illustrated in FIGS. 9A-9G, it can be considered that, after the completion of an operation of reading out the signal of a pixel at the processing illustrated in FIG. 9G, the operation is returned to the state of FIG. 9A and an accumulation operation is started. However, when the operation of the in-surface synchronous electronic shutter is performed, the start and finish times of a period of accumulating charges in pixels becomes the same times in all rows, but the read-out of signals from the pixels is performed by the row. Consequently, if the operation illustrated in FIGS. 9B-9G is repeated, then the time period of from the state illustrated in FIG. 9G to the state illustrated in FIG. 9B changes depending on rows of the pixels. If the time period changes, then dark current components accumulated in accumulation portions during the periods of from the state illustrated in FIG. 9G to the state illustrated in FIG. 9B change dependently on rows, and consequently the influences of the dark currents changes dependently on rows. Furthermore, not only the dark currents, but also the influences of lights entering the pixel region during the period change dependently on rows. Consequently, it is considerable that the brightness of an image obtained as a result changes dependently on rows. For this reason, it is considerable that false signals depending on the brightness of an object are generated, and the degradation of an image quality is feared.